The encoding of the temporal structure of sounds created in the brainstem auditory system is thought to underlie the representations of the units of speech and the locations of sound sources in the auditory cortex. Brainstem auditory neurons show several specializations that allow them to maintain and transmit precisely the timing information contained in sounds. One of the most important of these adaptations is synaptic glutamate receptors of the AMPA subtype with very rapid desensitization kinetics. The AMPA receptor structure that allows very rapid kinetics also allows high permeability of the receptor-gated ion channel to calcium. To regulate their intracellular calcium concentration within the narrow limits consistent with normal cellular function, auditory neurons employ several calcium- regulating mechanisms, including high levels of the calcium- binding protein calretinin. The characteristic features of AMPA receptors in auditory neurons develop after the onset of spontaneous electrical activity and synaptic function in the auditory pathway. During the same period in development, calretinin becomes localized beneath the cell membrane of auditory neurons, greatly enhancing this molecule's calcium- buffering capacity. The proposed experiments, grouped into two specific aims, will make manipulations of synaptic input to developing auditory neurons 1) in vivo by early deafening or pharmacologic blockade of activity, and 2) in vitro by synaptic pairing of auditory neurons with either auditory or non-auditory partners under different levels of spontaneous activity. The effects of these different developmental environments on AMPA receptor structure and function will be assessed by mRNA analysis, immunology, and electrophysiology. The effects on localization of calretinin will be assessed by quantitative immunohistofluorescence. These experiments will characterize the interacting developmental programs of AMPA receptors and calretinin and assess the influence of afferent synaptic connections and activity on the development of the specialized information-processing and calcium-regulating capacities of brainstem auditory neurons.